Well, no, we know the magnitude of how many stars are in the galaxy. And we know the Federation's aware of a couple hundred humanoid lifeforms because we've seen them. And we also know that humanoids are a tiny minority of the galaxy's population; Spock said so. What needs explaining?
I meant in real life. In real life we don't know what aliens look like if they exist. And there are theories that they could be completely exotic creatures we don't even recognize as life and there are theories that they could be painfully mundane because evolution and "survival of the fittest" only allows for so many viable forms.
People complained about Star Trek mostly showing humanoid aliens, because many prefer the explanation of the "so exotic we don't even recognize as life" theory and said Star Trek is unrealistic because of it.
However we won't know what's the score with that IRL until/if we meet real aliens.